Adaptive Therapy With Capecitabine for Treatment of Metastatic ER Positive, HER2 Negative Breast Cancer

Inclusion Criteria: * Age ≥ 18 years * Histological confirmation of estrogen-receptor positive (ER+), HER2-negative overexpression or amplification negative as per American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines, metastatic breast cancer * Measurable disease. Bone only disease allowed if associated with soft tissue component that is measurable by Response Evaluation Criteria is Solid Tumors (RECIST) 1.1 criteria * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2 * Hemoglobin ≥ 9.0 g/dL (obtained ≤ 14 days prior to registration), no transfusions allowed ≤ 14 days prior to registration * Absolute neutrophil count (ANC) ≥ 1500/mm\^3 (obtained ≤ 14 days prior to registration) * Platelet count ≥ 100,000/mm\^3 (obtained ≤ 14 days prior to registration) * Total bilirubin ≤ 1.5 x upper limit of normal (ULN) (obtained ≤ 14 days prior to registration) * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 x ULN (≤ 5 x ULN for patients with liver involvement) (obtained ≤ 14 days prior to registration) * Prothrombin time (PT)/international normalized ratio (INR)/activated partial thromboplastin time (aPTT) ≤ 1.5 x ULN OR if patient is receiving anticoagulant therapy and INR or aPTT is within target range of therapy (obtained ≤ 14 days prior to registration) * Calculated creatinine clearance ≥ 45 ml/min using the Cockcroft-Gault formula (obtained ≤ 14 days prior to registration) * Negative serum …